whatThe world has a true answer, left or right, and a line of people decide one at a time. Each holds a private hint that points true seven times in ten, and each can see every choice made before them, though never the hints behind those choices. Everyone reasons perfectly. Choices are drawn as leaning strokes; the underline marks where the herd formed. Click to reveal the hidden hints, ringed wherever they disagree with the choice made above them.

whyThe claim: rationality herds. Once the visible choices lean by just two, that lean outweighs any single hint, so every later person rationally discards their own evidence and follows -- measured here, the herd forms in every run, on average by the fourth chooser, and after the lock people contradict their own hint at exactly the hint's noise rate, three in ten. And roughly one herd in six locks onto the WRONG answer: a hundred perfectly rational people filing onto a mistake, each silently holding evidence against the choice they make. No fool anywhere in the line.

howThe Bikhchandani-Hirshleifer-Welch urn, run as a replaying queue. The rational rule reduces to: follow the visible lean when it reaches two, otherwise follow your hint. The truth is disclosed as an arrowhead when each run ends, and the tally counts herds and wrong herds across runs. One integer seeds every deal, so the same seed deals the same line.